1. Field of the Invention
The present invention relates to a communication device which is preferably used in two or more communication systems which use carrier signals of the same frequency band. More particularly, the present invention relates to a communication device which includes an RFID (Radio Frequency Identification) system antenna and a mobile communication system antenna.
2. Description of the Related Art
An RFID system which transmits and receives predetermined information between an RFID reader/writer and an RFID tag is known. The RFID reader/writer and the RFID tag have RFID IC chips (wireless IC chips) which process high frequency signals, and antennas which transmit and receive the high frequency signals.
The RFID system is generally an HF band RFID system which uses a 13 MHz band or a UHF band RFID system which uses a 900 MHz band. The UHF band RFID system in particular has a long communication distance and can collectively read a plurality of tags, and therefore is regarded as a viable goods management system.
For a goods management RFID system for process management and history management of electronic devices such as portable communication terminals, UHF band RFID systems which are disclosed in International Publication Nos. 2009/011154 and 2009/128437 and use a ground conductor of a printed circuit board as a radiating element are useful.
FIG. 7A is a plan view of a wireless IC device disclosed in International Publication No. 2009/011154, and FIG. 7B is a cross-sectional view of the wireless IC device. A ground conductor 21 is provided on a surface of a printed circuit board 20 of this wireless IC device. A cutout 21a is formed in one side portion of the ground conductor 21 to provide a loop electrode 31. Input/output terminal electrodes of a wireless IC chip 5 are electrically connected to connection electrodes 31a and 31b through bumps 8. Further, connection electrodes 31c and 31d are formed on the surface of the printed circuit board 20. Mounted terminal electrodes of the wireless IC chip 5 are electrically connected to the connection electrodes 31c and 31d through the bumps 8. The loop electrode 31 electromagnetically couples to the ground conductor 21 and functions as a matching circuit, and the ground conductor 21 functions as a UHF band RFID radiating element.
However, when a UHF band RFID system which uses a ground conductor of a printed circuit board as a radiating element is applied to a communication terminal device which uses a UHF band for mobile communication such as a GSM (registered trademark) (Global System for Mobile Communications) 900, total radiated power (TRP) upon transmission or total isotropic sensitivity (TIS) upon reception in the GSM (registered trademark) system deteriorates.
According to this phenomenon, a resonance point of an RFID tag appears at 900 MHz in a frequency band of a mobile communication antenna (a frequency band used by a mobile communication system). Therefore, it is predicted that the resonance point makes it difficult for the mobile communication antenna to receive communication signals of the mobile communication system.
FIG. 8A is a view illustrating a relationship between a communication frequency and a maximum communicable distance of a conventional RFID tag. FIG. 8B is a view illustrating frequency characteristics of insertion loss I.L. and return loss R.L. of a mobile communication antenna. A resonance frequency of an antenna of an RFID tag matches 900 MHz used by the RFID system. Therefore, as illustrated in FIG. 8A, a maximum communicable distance comes to a peak at 900 MHz and decreases even if the frequency becomes higher or lower. Because the resonance frequency of the antenna of this RFID tag is 900 MHz, as illustrated in FIG. 8B, a resonance point P appears at 900 MHz in the frequency band of the mobile communication antenna (the frequency band used by the mobile communication system). As a result, total radiated power (TRP) upon transmission and total isotropic sensitivity (TIS) upon reception in the mobile communication system deteriorate.